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Axial leaded components are inserted before radial leaded components because, typically, axial components are smaller overall than radial components. Because the associated tooling size is also smaller, inserting axial components prior to the radial components facilitates the stuffing of higher-density circuit boards. 40.2.5.1.3 Radial Leaded Components. After DIP and axial leaded components, the radial components are inserted into the circuit board. Radial components can vary greatly in size, shape, height, and weight (see Fig. 40.5). The lead finishes types and thicknesses are identical to those of axial leaded components. The possible exceptions are hermetically sealed components, such as active Si chip devices in TO-5 cans, optoelectronic packages, and relays. Such devices typically have low-expansion, Fe-based alloy leads that allow for them to be sealed into the header with a glass-to-metal joint. Although some device manufacturers use a Sn-Pb or 100 percent Sn finish over an electroplated Ni (solderable) layer, other suppliers prefer an Au (protective) finish over the Ni (solderable) layer to provide adequate solderability. In such cases, the Au layer is removed by hot solder dipping in order to prevent Au embrittlement of the circuit board interconnection.
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FIGURE 40.5 Radial leaded components: (a) individually; (b) in taped reels. (Courtesy of Sandia National Laboratories and Universal Instruments.)
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Radial leaded components are taped together at the leads. Radial insertion machines remove the components from the tape and insert them in sequence on to the printed circuit board using a shuttle system similar to that used for other components. The insertion machine tooling for radial components is designed to avoid previously inserted DIP and axial components. 40.2.5.1.4 Odd-Form Components. The odd-form components are the last devices to be assembled on the circuit board. Odd-form components, by definition, are those packages that are not readily addressed by automated assembly because (a) they are used at insufficient volumes on the product to justify machine space or (b) they are of a geometry (shape or size) that lacks the customer demand to justify the machine manufacturer to provide off-the-shelf tooling for them. High-power applications often require a variety of odd-form components, including simply oversized DIP, axial leaded packages, or radial leaded packages. Also, odd-form components include transformers, switches, relays, and connectors. Several of these packages are shown in Fig. 40.6. Unusual package sizes and geometries are often accompanied by unusual lead configurations and materials. It is not uncommon to have Cu- and Ni-based alloy leads, or even leads made of refractory materials such as Mo and Ni.The leads may be round, square, or in the form of ribbons. Irrespective of the lead material or nontypical geometry (which must have the hole geometry addressed at the circuit board design step), it is always required that the leads have adequate solderability. Leads are typically plated with solderable layers (Cu or Ni) and
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FIGURE 40.6 Examples of odd-form, through-hole components and a circuit board assembly. (Courtesy of Universal Instruments.)
protective finishes (Au, Sn, Sn-Pb, etc.) very much like those used on traditional components to support the soldering process. 40.2.5.2 Insertion Equipment. Because the demand for through-hole circuit boards has remained strong, insertion equipment capabilities have steadily improved. Brushless servomotors and state-of-the-art motion controllers and sensors have replaced most pneumatic drive assemblies and bulky (and slow) mechanical switches and relays, respectively. Some equipment options include the capability to test parts electrically prior to insertion. At insertion rates of up to 40,000 components per hour (CPH), defect rates are a couple of hundred parts per million or lower for the common axial and radial leaded packages. Interchangeable tooling has allowed the automated insertion of many odd-form components, as well. The change to Pb-free solders does not have a direct impact on insertion equipment technology. 40.2.6 Automated Soldering and Wave Soldering The most commonly used process for the soldering of through-hole and mixed (through-hole and surface-mount) circuit boards is wave soldering. The wave-soldering process is shown schematically in Fig. 40.7. The populated (or stuffed ) circuit board is secured to a conveyor belt. The conveyor belt carries the board through the fluxer, then the preheat stage, and lastly on to the molten solder wave. Interestingly, one of the most critical steps in the wave-soldering process is the application of the flux. Manual fluxing has given way to more precise, automated equipment in order to improve yields. The advantage is better control of the quantity of flux on the circuit board as it enters the wave because the flux controls both the entry and exit geometries of the molten solder wave, which in turn are instrumental toward minimizing skips, bridges, or icicle defects. Flux can be applied by spraying techniques or by passing the circuit board through the foam or suds of the flux. The latter technique is provided by the foam fluxer.
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